Ok heres all the Theory youll ever need ....for soloing.
Learn your MODES and the sound they create ...
Look how that all realte to the major scale.
Lets take a C Maj or Scale and let see how the modes relate to it
cdefgabc < Ionian Mode / Major Scale
defgabcd < Dorian Mode
efgabcde < Phrygian Mode
fgabcdef < Lydan Mode
gabcdefg < Mixolydian Mode
abcdefga <Aeolian Mode
bcdefgab < Locrian Mode
All of these scales work over C Major....A minor and a million other chords that contain the notes of the C Major scale.
To over simplify the modes lets go furthur.
The modes are based off the notes of the major scale in the key you are playing in.
So lets use C again as an example.
If you start playing the notes of the C major scale from the 2nd note of the scale you get the D Dorian scale, the scale is named for the note it starts on and Dorian is its Modal Tonality. Play a D5 chord and play the dorian scale over it and you will see the type of sound it has over the scale.
The rest of the modes of the c major scale work in the same way.
Phrygian and Aeolian are the choice for metal. Phrygian has that cool Middle Estern ot Neo Classical sound. It works great with the Harmonic Minor scale.
Heres your crash course in how to learn your modes
Hope this helps
Da Fukn Guru